Evaluation of the short-term benefits of smoking cessation in adolescents is important because the knowledge of these benefits may enhance treatment outcomes for young persons. To date, most research on the medical benefits of smoking cessation has focused on adult smokers (i.e., reduced risk for lung cancer, hear disease). Unfortunately, these health benefits may not be meaningful enough for adolescents to quit smoking. Thus the current proposal seeks to determine a potentially important benefit of smoking cessation in adolescents. Specifically, we will examine effects of smoking cessation on bone turnover in adolescent girls and boys. This information may eventually be useful for treatment of adolescent smokers, especially if it can be presented in terms of maximal bone strength and height potential. The primary aims of this study are: 1) To determine the effects of smoking cessation on biochemical markers of bone turnover in adolescents; and 2) To examine the potential mechanisms by which smoking could affect bone turnover by measuring plasma cotinine concentrations and hormone profiles. We hypothesize that smoking cessation will increase markers of bone formation in adolescent boys during their rapid growth rate. We also hypothesize that smoking cessation will decrease markers of bone resorption in adolescent girls who have completed their rapid growth phase. Additionally, we anticipate that the change in biochemical markers of bone turnover in boys or girls will be correlated with a decrease in plasma cotinine concentrations. Study subjects will be recruited primarily from an ongoing study of acute nicotine abstinence in adolescent smokers (RO1 HD 37688; P.I. Krishnan-Sarin) being conducted at Yale University School of Medicine. The present proposal will examine four groups of smokers (N=60): boys versus girls, who either quit smoking for three months or who continue smoking. Blood and urine for markers of bone formation (bone specific alkaline phosphatase, osteocalcin and C-terminal pro-collagen peptide) and resorption (cross-linked amino-terminal and carboxy-terminal telopeptides of collagen and deoxypyridinoline free cross-links) will be collected on all smokers at baseline, and again at the three month up visit. Anthropomorphic measurements (e.g., height and weight) will be assessed at baseline, three months, and again at one year.